The present invention relates to a power supply circuit and a disk drive provided with the power supply circuit.
A typical example of a conventional power supply circuit for supplying electric power to a largely variable power-consuming load is a power supply circuit used for disk drives capable of storing and retrieving information held on disk media.
Referring to FIGS. 1 to 3, the conventional power circuit used for a portable type MD (magnetic disk) recording and reproducing device will be described below.
In FIGS. 1 to 3, there is shown a recording and reproducing means 1 composed of a pickup, a pickup servo, a pickup drive and radio-frequency amplifier, which writes data on a disk by a high-power laser beam generated by the pickup and reads data from a disk by a micro-power laser beam generated therefrom.
There is shown a signal processing circuit 2 for transferring data from and to the recording and reproducing means 1 and processing the data by companding (compressing and expanding), which circuit includes a memory 2a in which data compressed when recording by the signal processing circuit 2 and compressed data read from the disk are stored. While the data is compressed and stored by the signal processing circuit 2 with the memory 2a, the pickup has nothing to do and, therefore, stops emission of a laser beam to the disk and the pickup servo and the drive also stop. This realizes a considerable saving in electric power since the load while the pickup halts is smaller than a half of the load while the pickup being in operation.
Numeral 3 denotes a microcomputer for control of an entire system of the MD recording and reproducing device and numeral 4 denotes a signal converting circuit whereby data input through an input terminal is converted from analog to digital and transferred to the signal processing circuit 2 and data output from the signal processing circuit 2 is converted from digital to analog and transferred to an output terminal.
A first voltage increasing/decreasing circuit 5a (up-down type DC-DC converter) 5a is used for increasing or decreasing a voltage from a power supply circuit (to be described later) to 2.5 volts and a second voltage increasing/decreasing circuit (up-down type DC-DC converter) 5b is used for increasing and decreasing a voltage from the power supply circuit (to be described later) to 2.8 volts. The first voltage increasing/decreasing circuit (DC-DC converter circuit) 5a is connected to a circuit requiring the power supply of 2.5 volts while the second voltage increasing/decreasing circuit (DC-DC converter circuit) 5b is connected to a circuit requiring the power supply of 2.8 volts. The decreasing or increasing of voltage transferred from the power supply circuit (to be described later) is needed because the voltage may not be constant and is more and less than a required value.
There is also shown a dry battery 6a, a rechargeable battery 6b capable of charging and discharging and an AC adapter 6c for converting an alternating current to a direct current. These components are selectively attached and detached by a user of the device. The magnitudes of the output voltages of the respective power sources, i.e., the AC adapter 6c, the rechargeable battery 6c and the dry battery 6a become smaller in the described order. The dry battery 6a are inserted in a battery case 10 as illustrated in FIGS. 2A, 2B and the battery case 10 is then fitted at its male terminals in corresponding female terminals of the device body 20 as illustrated in FIG. 3. The battery case 10 is electrically connected to the device body 20 by turning a tightening screw. Because current from the dry battery 6a is constant in line to the first DC-DC converter circuit 5a and the second DC-DC converter circuit 5b, power loss due to resistance of a transmission line from the dry battery 6a to a load cannot be neglected.
Switches 7a, 7b and 7c are used for turning on the power supply from the dry battery 6a, the rechargeable battery 6b and the AC adapter 6c respectively. The switches 7a, 7b, 7c are turned on and off according to control signals of the microcomputer 3 to transmit the power to the first DC-DC converter circuit 5a, the second DC-DC converter circuit 5b and a voltage increasing circuit 8 to be described later. The switches 7a, 7b and 7c may be of, e.g., a power MOSFET (metal-oxide-semiconductor transistor) having small on-resistance.
Diodes 7d, 7e, 7f are disposed in parallel with the switches 7a, 7b, 7c respectively. When any of the power sources was disconnected by the user, another power supply whose output voltage is next in magnitude to that of the removed power supply is selected to output a voltage through the diode corresponding to the selected power supply.
A voltage increasing circuit 8 is used for increasing an input voltage to 2.8 volts. This circuit is connected to the switches 7a, 7b and 7c respectively and can receive power from the dry battery 6a, the rechargeable battery 6b and the AC adapter 6c. The power from the dry battery 6a is increased to 2.8 volts and further transmitted. On the other hand, the power generated by the rechargeable battery 6b or the AC adapter 6c is transmitted without being increased since the output voltage is higher than 2.8 volts.
A voltage decreasing circuit 9 is used for decreasing the power voltage fed from the voltage increasing circuit 8 to 2.5 volts that is then used for driving the microcomputer 3. The voltage increasing circuit 8 and the voltage degreasing circuit 9 is provided to drive the microcomputer 3 first without fail when a start (power) key (not shown) of the MD recording and reproducing device is turned on. Namely, the first voltage increasing and decreasing circuit 5a is controlled by the microcomputer 3 and, therefore, it cannot drive the microcomputer 3.
In the above-described MD recording and reproducing device, the microcomputer 3 is unconditionally driven when a start key (not shown) on the device is turned on, and it recognizes the power sources attached to the device body by measuring voltage of each power supply and selects one of the attached power sources according to the priority in the order of the AC adapter 6c, the rechargeable battery 6b and the dry battery 6a. The microcomputer 3 then turns on a switch (7a, 7b or 7c) corresponding to the selected power supply to transmit power therefrom to the device. The microcomputer 3 determines the output voltage of the power supply when the output increases and when data is written or read by the device, compares the measured voltage with a warning level and a life-end level preset for each power supply mode and gives a warning indicating the battery approaching to the life end level or stops the power supply mode when its output reaches the life end level.
The warning voltage level of a dry battery 6a when writing data on a disk is 1.0 volts and its life-end level is 0.9 volts. The warning level of the battery 6a when reading data from a disk is 1.0 volts and the life-limit level is 0.8 volts. The warning level and the life-limit level of the battery 6a for the writing operation are set higher than those for the reading operation. This is because the load during the recording operation is higher than the load during the reproducing operation and, furthermore, the data recording operation is not completed by writing main data on a magnetic disk (MD) and continues until UTOC (User Table Of Contents) data relating to the main data is recorded in a UTOC area. Namely, the recording operation requires additional power energy for writing UTOC data for all the main data written during the operation.
The above-described conventional device involves the following drawbacks:
1. The microcomputer may have a hang-up or a dead halt.
When the device was de-energized by eventually disconnecting a power supply during the operation of the device, the microcomputer turns on a switch of a next designated power supply to immediately start the supply of power through a diode for protecting the voltage from dropping in the meantime. However, there may still be a risk of hanging-up of the microcomputer from the voltage drop across the diode if the output of the next designated power supply should be at the life-end level.
2. The power supply circuit is very expensive.
A relay type switch cannot be used for applications where high-speed switching is required. A bipolar type transistor switch requires a base current associated with a power loss and is unsuited to use in applications where power saving is desired. Consequently, power MOSFET switches being drivable with a potential applied thereto are necessarily used for the MD recording and reproducing device. However, the power MOSFET switch having small on-resistance is very expensive and must be provide done for each of the respective power supply circuits. The use of plural expensive switches irrevocably increases the manufacturing cost of the device.
3. The power supply circuit has a large loss of power.
Although a power MOSFET switch having a less power loss is applied, it has still a negligible power loss due to on-resistance in particular when operating under a large load. This serves as an obstacle to realizing the power-saving device. When the device is powered from a power supply consisting a small number of dry batteries, it is necessary to provide a voltage increasing-decreasing circuit consisting of an up-down type DC-DC converter for increasing the output voltage to the level preset for the load to prevent the output voltage from dropping lower than the level required by the load. In this case, a power loss in a voltage decreasing circuit is about 10% of the power while a power loss in a voltage increasing-decreasing circuit is about 30%. Namely, the power supply circuit has a large power loss.
A dry battery power supply may be used with a voltage decreasing circuit only but requires in this case the use of at least four dry batteries. The use of increased number of dry batteries is of course unfavorable.
4. The operation life of a dry battery is short.
A dry battery is selected to power the device when the AC adapter is not attached to the device and the rechargeable battery is attached but has a decreased output lower than the preset level. In this instance, a charge in the rechargeable battery cannot be used and the battery power supply must work only with a large external load, so the battery may quickly reach the life-end level because of the large loss of the charge caused in the power supply circuit and an increased drop of its voltage caused due to an increased internal resistance of the battery.
5. A plurality of dry batteries must be used.
The output voltage drop of a dry battery may arise from increasing internal resistance (not due to the starting voltage drop). Since the internal resistance increases with an increase in the external load, the output of the battery may quickly decrease to the life-end level. It is therefore required to use an increased number of dry batteries to elongate the operation time of the device. This is unfavorable in particular for portable type devices.